The present invention relates generally to communications and more particularly to a system and method for using an existing pilot signal in an OFDM modulated transmitted signal in a communication system, and used by the receiving end, in conjunction with multiple antenna elements. The receiver can implement a separation process known as spatial filtering, or also referred to herein as smart antenna.
Wireless networks and in particular wireless broadband networks having multiple information channels are subject to certain types of typical problems such as inter-channel interference and a limited bandwidth per information channel. For example, broadband wireless networks can use cellular and frequency-reuse schemes to enable the number of simultaneous users to be increased for a given service area within a given allocated spectrum. In such a broadband wireless network, inter-cell interference constrains the aggregated bandwidth of the network.
In sum, a need exists for an improved system and method that can significantly reduce the amount of spectrum allocated to communicate a given amount of data or that can significantly increase the amount of data for a given amount of allocated spectrum.
Orthogonal frequency divisional multiplexing (OFDM) multiple access is one type of modulation scheme used in broadband networks. OFDM modulation is based on multiple carrier frequencies within a given allocated spectrum. Although most of the carrier frequencies carry information, some of the carrier frequencies are “overhead” that are used for management, estimation and calibration of the OFDM channel (i.e., the allocated spectrum used for the OFDM modulation). These “overhead” carrier frequencies are referred to herein as pilots that can carry pilot signals. Typically, these pilot signals are used to improve the frequency offset, phase noise, amplitude normalization, accurate symbol timing, and/or group delay at the receiver. These pilot signals serve as a channel estimator and are typically modulated by a single known code that is used by the entire network.
OFDM modulation can be implemented in various ways including the various standards based on such protocols as 802.11a, DVB-T, BWIF, IEEE 802.16 and others. For example, the IEEE 802.11a standard (also referred to as the WiFi standard) specifies the use of 52 carriers, four of which are designated as pilots and are spread across the allocated spectrum.
As described below, these existing OFDM pilots can be used to reduce the amount allocated spectrum to communicate a given amount of data or to increase the amount of data for a given amount of allocated spectrum.